Many wireless devices are constrained to a small volume, and therefore use Electrically-Small Antennas (ESAs). An ESA has physical dimensions that are small relative to the free-space wavelength λ. One example definition regards an antenna as electrically small if it is able to fit in a sphere of radius λ/2π.
Electrically-Small Antennas are described, for example, by Wheeler, in “Fundamental Limitations of Small Antennas,” Proceedings of The IRE, volume 35, issue 12, December, 1947, pages 1479-1484; by Wheeler, in “The Radiansphere Around a Small Antenna,” Proceedings of The IRE, volume 47, issue 8, August, 1959, pages 1325-1331; and by McLean, in “A Re-Examination of the Fundamental Limits on The Radiation Q of Electrically Small Antennas,” IEEE Transactions on Antennas and Propagation, volume 44, issue 5, May, 1996, pages 672-675, which are all incorporated herein by reference.